1. Field of the Invention
The invention relates to a circuit layout for an infrared room surveillance detector comprising a pyroelement sensor, and more particularly, a circuit layout operated in current mode and containing an operational amplifier with an output signal conducted to a threshold value comparator stage.
2. Description of the Related Technology
Detector circuits used in combination with pyroelements serving as infrared sensors, usually are detectors operated in the voltage mode. Detectors operated in the voltage mode have a high impedance required by the further processing of the detector output signal by, for example, a threshold value comparator stage. The disadvantage of such detectors operating in the voltage mode is that detector sensitivity is inadequate and, in particular, that detector amplification decreases at high frequencies. The significance of this is that the degree of amplification and thus the output voltage varies with the frequency.
Use of a detector operating in the current mode in combination with a pyroelement serving as an infrared sensor has been proposed. This leads to a higher detector sensitivity. The detector has a constant amplification factor over a relatively broad frequency band including higher frequencies representing a particular advantage in actual use. The disadvantage of such a detector operating in the current mode is that is has a relatively low impedance. It has therefore been proposed in connection with the present invention to insert between the pyroelement and an operational amplifier an impedance converter in the form of a field effect transistor (FET). This provides the high impedance required, but the use of an impedance converter or an FET has the considerable disadvantage that the detector becomes appreciably more susceptible to external electromagnetic interference and the electromagnetic compatibility (EMV) required by the receiving authority cannot be assured.